The Diverse Roles of Extracellular Leucine-rich Repeat-containing Receptor-like Proteins in Plants

Plant cells use various types of cell surface receptor molecules to sense extracellular signals and modulate cell-to-cell communication in many biological processes. Extracellular leucine-rich repeat (eLRR) receptor-like proteins (RLPs) represent an important class of such cell surface receptors. RLPs differ from receptor-like kinases (RLKs), which compose the largest class of cell surface receptors in many plant species, because they lack a cytoplasmic kinase domain. RLPs play roles in both developmental processes and disease resistance. A total of 57 RLP encoding genes has been identified in Arabidopsis. Two of them, CLAVATA2 (CLV2) and Too Many Mouths (TMM) have a function in meristem maintenance and stomatal distribution, respectively, whereas few others act in basal defense against pathogens. Although the function of most RLPs in Arabidopsis remains unclear, considerable progress has been made in understanding RLP functioning and signaling over the years. This review focuses on the function of RLPs in plants. Furthermore, the function of distinct RLP domains and the role of conserved residues important for perception and ligand specificity are discussed. The role of RLP proteins in multimeric complexes to sense biotic and abiotic extracellular signals is also addressed.